Re: [Xen-devel] [PATCH 5/6] x86/AMD: Fix handling of FPU pointer on Zen hardware

[Roger Pau Monné <roger.pau@xxxxxxxxxx>]

On Fri, Dec 28, 2018 at 12:39:35PM +0000, Andrew Cooper wrote:
> AMD hardware before Zen doesn't safe/restore the FPU error pointers
> unless an unmasked FPU exception is pending.  Zen processors have a
> feature bit indicating that this (mis)behaviour no longer exists.
> 
> Express the common logic in terms of cpu_bug_fpu_err_ptr as Hygon
> processors (being Zen derivatives) won't inherit this behaviour.
> 
> While at it, fix a performance issue with the workaround, which I should
> have noticed by now.  Looking at the FPU state slows the context switch
> path down, as it is a moderately complicated unpredictable condition
> which will evaluate to true for all 64bit OSes and most 32bit ones.
> 
> Leave the sole condition being the easily-predictable
> cpu_bug_fpu_err_ptr as the asm sequence is needed in ~100% of cases
> these days on affected hardware.

I'm not that familiar with the FPU registers, but I assume current
code checks whether there's an exception pending before decoding
whether the error pointers should be cleared?

> Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>

Apart from the question above, LGTM:

Reviewed-by: Roger Pau Monné <roger.pau@xxxxxxxxxx>

Thanks.

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